This invention relates to motor control circuits and to methods of controlling a multi phase electric motor.
Electric motors are becoming increasingly common in a diverse range of applications. It is known, for example, to provide an electric power steering (EPS) system of the kind comprising an input shaft, an output shaft, a torque sensor adapted to measure the torque in the input shaft, and an electric motor adapted to apply an assistance torque to the output shaft dependent upon the torque measured by the torque sensor.
A typical permanent magnet electric motor comprises a rotor that is magnetic, for example including a permanent magnet, and a stator including a plurality of phase windings on a yoke. Applying suitable voltages across each of the phase windings causes current to flow through the windings, generating a current flux vector in the air gap between the stator and the rotor. This flux interacts with the magnetic field of the rotor magnets to cause the rotor to rotate to a point of equilibrium in which the current vector is aligned with the axis of the rotor magnetic field.
To cause the rotor to turn continuously, the current passed through the windings must be varied in a sequence. This causes the current vector to rotate. This can be achieved by modulating the voltages across each winding under the control of a motor drive circuit.
The torque developed in the motor is dependent firstly on the current passing through the windings, in a generally linear manner, and secondly on the phase of the current relative to the flux due to the rotor magnets.
When the rotor is stationary, the greatest torque is developed when the current vector is in quadrature with the rotor flux vector; no torque will be developed when the current is in phase with the rotor flux. For this reason, motors are generally controlled so as to keep the current in quadrature with the rotor flux. However, as rotor speed increases the back emf that is produced in the coils also increases. This limits the torque that can be produced. It is known to advance the phase of the current with respect to the rotor flux, which can increase the torque available under certain circumstances.
Furthermore, the maximum rotational speed of such a motor is dependent upon the voltage at the motor phase terminals—this voltage must be greater than the back EMF generated by the motor in order to generate a torque. The back EMF increases generally linearly as a function of speed of the motor. For a battery-powered motor such as would be used in an EPS system, the maximum motor speed is therefore indirectly governed by the battery voltage. This can also be countered by advancing the phase of the current relative to the motor, which produces a current component that is aligned with the magnetic field and can act to reduce, or weaken, the field, in turn reducing the back emf at a given rotor speed. With the field reduced, the motor speed can be increased.